1. Field of the Invention
The present invention relates to piezoelectric devices, and in particular relates to piezoelectric devices that include a piezoelectric resonator such as a BAW resonator.
2. Description of the Related Art
In the related art, piezoelectric resonators are known to include a piezoelectric vibrating portion that is acoustically isolated from a substrate, such as a BAW resonator.
There is a problem with this type of piezoelectric device that includes a piezoelectric resonator in that, due to the polarization of a piezoelectric thin film, the piezoelectric resonator is not displaced linearly with voltage, but rather is displaced non-linearly. A known example of a piezoelectric device that solves this problem is described in Japanese Unexamined Patent Application Publication No. 2007-6495. In the piezoelectric device described in Japanese Unexamined Patent Application Publication No. 2007-6495, as illustrated in FIG. 9, a first BAW resonator 172 and a second BAW resonator 174 are connected in parallel with each other between a first electrical terminal 176 and a second electrical terminal 178. Arrow 172R, which indicates the polarization direction of the first BAW resonator 172, and arrow 174R, which indicates the polarization direction of the second BAW resonator 174, point in opposite directions to each other when viewed from the first electrical terminal 176. Accordingly, in this piezoelectric device, it is possible to cancel out the non-linear effect that occurs due to the polarization direction dependence.
In general, resonance characteristics of a thickness extensional vibration mode are employed in a BAW resonator, but sometimes ripples are generated in a band due to a transverse vibration mode, which is not desirable in normal operation. However, it is desirable that piezoelectric resonators having the same impedance be connected in parallel with each other in order to suppress the non-linear effect. In such a case, the frequency of ripples and the strength of ripples, which ripples are generated by the individual piezoelectric resonators, are the same. Therefore, there has been a problem in that the ripples reinforce each other and the characteristics of the piezoelectric resonators are degraded.